This is a poor comparison, BitTorrent has lots of obvious legal uses even if it is popular with pirates. It actually gets used extensively for those legal purposes. Products like LimeWire or Napster explicitly marketed for piracy reasons are another matter. It is possible to use BT without breaking the law.
Maybe a decade ago when I worked at a game studio our IT department got mad at me for being connected to a torrent tracker. What was the tracker for? Patches for one of our games, because the updater used BT.
So what Percentage of BitTorrent Traffic would need to be "illegal" for it to fall under the classification of "obvious violation of US law" or "primarily used for illegal purposes" thus justifying targeting the technology instead of the individuals using the technology?
I have a feeling in this instance your bias is in favor of allowing bittorrent to exist because you have a personal use for that technology that is not illegal, and you do not have a personal use for the Tornado Cash technology, thus you have no ability to see legal uses for that tech.
I have found it to be an exceedingly rare trait for people to be able to externalize, and understand other people's worldviews. If they personally do not need, desire, or have a use for X, then they have no problems with the government clamping down, regulating or banning it. Never coming to the wider understanding that the government may (and likely will) turn it gaze to them.
Sounds like a apt comparison to Tornado Cash then, as exactly the same applies with it. It's used for obviously legal use cases too. Just because some groups use it for less legal things, doesn't mean the thing itself should be illegal.
Maybe a decade ago when I worked at a game studio our IT department got mad at me for being connected to a torrent tracker. What was the tracker for? Patches for one of our games, because the updater used BT.